Diastereoselective synthesis of potent antimalarial cis-β-lactam agents through a [ 2 + 2 ] cycloaddition of chiral imines with a chiral ketene

The effect of double asymmetric induction for the s ynthesis of new cisβ lactams by [2+2] cycloaddition reactions of chiral imines with a chiral ketene was investiga ted. The cycloaddition reaction was found to be tot ally diastereoselective leading exclusively to the forma tion of the cis-β-lactam derivatives. The newly synthesized cycloadducts were evaluated for their antimalarial activities against Plasmodium falciparum K14 resistant strain with moderate to excellent IC 50 values varying from 8 to 50 μM. Of the fifteen β-lactams tested, four showed IC 50 ≤ 11 μM. Kywords: β-lactam, chiral imine, chiral ketene, Staudinger re action, Diastereoselective, Antimalarial activity.


Introduction
Malaria has been a serious public disease for many decades. The development of resistance by plasmodium falciparum to drugs such as chloroquine and quinine requires the discovery and development of new drugs (1). A new diterpenoid β-lactam alkaloid showing potent antimalarial activity has been isolated from marine sponge Hymeniacidonsp by Rodriguez and his coworker (2). Due to spread of resistance in the mosquito vector to currently available insecticides the control of malaria is becoming more complicated. Therefore, it is necessary to synthesize new classes of antimalarials (3), and to develop them as drugs with varied models of action to overcome the problem of resistance (4). The development of a novel class of antimalarials derived from β-lactam has initiated in recent years (5-6). β-Lactam derivatives with various functional groups have played an important role in antibacterial drugs and in medicinal chemistry (7). The β-lactam ring is an important structural element of the most widely employed β-lactam antibiotics family (8), which includes representative structural classes; penams, cephems, penems, monobactams, carbapenems, and trinems (9). In addition, β-lactams show many important non-antibiotic biological activities (10). They also have increasingly being used as valuable starting materials to develop new synthetic methodologies (11)(12). The constant need for potent and effective β-lactam antibiotics as well as more effective β-lactamase inhibitors has prompted synthetic organic and medicinal chemist to design new functionalized 2-azetidinones. There are a large number of synthetic methods for the preparation of βlactams (13), for which the [2+2] cycloaddition of ketenes with imines (the Staudinger reaction) is the most important method for constructing the 2-azetidinone ring (14).

General
All needed chemicals were purchased from Merck, Fluka and Acros. All reagents and solvents were dried prior to use according to standard methods. IR spectra were run on a Shimadzu FT-IR 8300 spectrophotometer. 1 H-NMR and 13 C-NMR spectra were recorded in DMSO-d 6 or CDCl 3 using a Bruker Avance DPX instrument ( 1 H-NMR 250 MHz, 13 C-NMR 62.9 MHz). Chemical shifts were reported in parts per million (δ) downfield from TMS. All of the coupling constants (J) are in hertz. The mass spectra were recorded on a Shimadzu GC-MS QP 1000 EX instrument. Elemental analyses were run on a Thermo Finnigan Flash EA-1112 series. Melting points were determined in open capillaries with Buchi 510 melting point apparatus. Thin-layer chromatography was carried out on silica gel F254 analytical sheets obtained from Fluka. Column chromatography was performed on Merck Kiesel gel (230-270 mesh).

General Procedure for Preparation of Schiff Bases 3a-j
A solution of N-tert-butoxycarbonyl-1, 2-ethanediamine (0.50 g, 3.10 mmol) in anhydrous CH 2 Cl 2 (25 mL) was treated with different aldehydes (3.10 mmol) in the presence of anhydrous MgSO 4 (6.00 g). The reaction mixture was stirred at room temp for 16 h, filtered and the solvent eliminated under vacuum to give crude Schiff bases 3a-j. They were used for next stage without further purification.

General procedure for the synthesis of monocyclic β-lactams 5a-o
A solution of acyl chloride (1.2 mmol) in dry CH 2 Cl 2 (10 mL) was slowly added to a solution of Schiff bases (1.0 mmol) and triethylamine (2 mmol) in CH 2 Cl 2 (15 mL) at -82 o C. The reaction mixture was then allowed to warm to room temperature, stirred overnight and then the solution was washed successively with HCl 1N (20 mL), saturated NaHCO 3 (20 mL), and brine (20 mL). Then, it was dried over Na 2 SO 4 and then filtered. The solvent was evaporated under reduced pressure to give the crude product. All β-Lactams were purified by recrystallization from EtOH, EtOAc except 5j which was purified by column chromatography ethyl acetate / petroleum ether (1:2).

T e r t -b u t y l 2 -( 3 -m e t h o x y -2 -o x o -4phenylazetidin-1-yl)ethylcarbamate (5o)
White General procedure for the deprotection of BOC protecting group A solution of β-lactams (5a-o) (0.5 mmol) in CH 2 Cl 2 (12 mL) was cooled to 0 °C and treated with TFA (3-5 mmol). After the addition, the cooling bath was removed and stirring was continued until total disappearance of the starting material (TLC). Then the solution was basified with 5% aqueous NaOH solution (pH = 10). The organic layer was separated, and the aqueous layer was extracted with CH 2 Cl 2 (2 × 30 mL). The combined extracts were washed with brine, dried over Na 2 SO 4 , and concentrated to give crude N-(2-aminoethyl) β-lactams 6a-o.

General procedure for antimalarial activity measurements
The chloroquine-resistant P. falciparum strain K14 (Southeast Asia) was cultured in vitro in complete medium consisting of RPMI 1640 (In Vitrogen) supplemented with 27.5 mM NaHCO 3 , 20 mg/L gentamycin, and 10% human serum (19). Parasites were grown at 37 °C in human O+ red blood cells at a 6% hematocrit under a 5% CO2, 10% O2 and 85% N2 atmosphere. Cultures were synchronized by sorbitol treatments (20). Stock solutions of lactam derivatives were prepared in sterile DMSO (10 mM) and later dilutions were with complete culture medium. Increasing concentrations of lactam derivatives (100 µL/ well, top concentration = 50 µM) were distributed in a 96-well plate; DMSO (0.5% vol/vol, top concentration) was distributed for control. Then, 100 µL from a culture containing > 95% ring (0-20 h postinvasion) at a 0.8% parasitemia and 3% haematocrit in complete medium was added per well along with 1.0 µCi of 3H-hypoxanthine with a specific activity of 14.1 Ci/mmol (Perkin-Elmer, Courtaboeuf, France). Parasites were grown for 42 h at 37 °C. Plates were then freezethawed and harvested on filters. Dried filters were moistened in scintillation liquid mixture (Microscint O; Perkin-Elmer) and counted in a Top Count Microbeta counter (Perkin-Elmer). Percentage growth inhibition was calculated from the parasite-associated radioactivity. 100% 3H-hypoxanthine incorporation was determined from a control grown in the absence of lactam derivatives. The concentration of drug giving 50% inhibition of label incorporation (IC50) was determined by nonlinear regression analysis of log-based dose-response curve (Riasmart; Packard). Each concentration was estimated from independent experiments in triplicate.

Results and discussion
A mixture of amine 1 which was prepared from ethylenediamine and di-tert-butyl dicarbonate ((BOC) 2 O) (15), and substituted benzaldehydes (2a-j) in anhydrous dichloromethane (DCM) and MgSO 4 was stirred at room temperature to give the crude imines 3a-j (Scheme 1). N-BOC protected β-lactams 5a-o were then synthesized by the Staudinger reaction. For this, to a mixture of imines 3a-j and triethylamine, substituted acetyl chlorides 4 were added dropwise at -82 º C to afford crude β-lactams 5a-o. These β-lactams were purified by recrystallization from either ethanol or ethyl acetate in moderate to good yields (Table 1).
The cycloadducts were characterized by spectral analysis. For 5b, the IR spectrum showed the characteristic absorption of a β-lactam carbonyl at 1749 cm −1 and the carbonyl signal of carbamate at 1696 cm −1 as well as the NH signal at 3359 cm −1 . The 1 H-NMR spectrum exhibited the t-butoxy protons at 1.46 ppm, the β-lactam ring H-3 and H-4 protons resonated as two doublets at 5.45 (j = 4.3) and 5.26 (j = 4.3) respectively. The cis and trans stereochemistries of 2-azetidinones were deduced from coupling constants of H-3 and H-4 (J 3,4 >4.0 Hz for the cis and J 3,4 <3.0 Hz for the trans stereoisomer) (16-17). The NH signal was exhibited at 4.96 ppm as a broad peak in CDCl 3 eliminated when 5b was vigorously shaken with D 2 O. The 13 C-NMR spectrum of 5b exhibited the C-3 and C-4 of β-lactam ring at 81.7, 60.2 respectively, C=O (β-lactam) at 165.0 and (C=O, BOC) at 155.5. Subsequent treatment of β-lactams 5a-o with trifluoroacetic acid (TFA) in dry CH 2 Cl 2 at room temperature afforded the deprotected N-(2-aminoethyl) β-lactams 6a-o (Table1).
Removal of the BOC residue resulted in removing of t-butoxy signal in 1 H-NMR and the appearance of the NH 2 peaks in IR spectra. The removal of the BOC was also confirmed by mass spectra and elemental analyses. Thus, for example the 1 H-NMR spectrum of 6a showed β-lactam H-3 and H-4 protons as two doublets at 5.60 ppm (J = 4.4 Hz) and 5.18 ppm (J = 4.4 Hz), respectively confirming the cis stereochemistry for these free amino β-lactam. The IR spectrum of this compound displayed NH 2 peaks at 3349, and 3417 and β-lactam carbonyl at 1734 cm -1 . The X-ray crystal structure of a paramethoxyphenyl derivative which is very close to 5d and 6d is shown in Figure 1.
The central β-lactam ring is almost planar. The methoxyphenyl ring is almost coplanar with β-lactam ring, whereas the tolyl ring is almost normal to it. The dihedral angle between β-lactam ring and O-bonded phenyl ring is 51.95 (18).

Biological activities Antibacterial and antifungal activities
Compounds 3a-j, 5a-o, 6a-o, were tested against S. aureus , E. coli, and C. albicans showing no activity below 125 µg/mL.In another assay, compounds 5a-i were tested against four American Type Culture Collection (ATCC) strains C. albicans (ATCC10261), A. flavus 11 CDCl 3 eliminated when 5b was vigorously shaken with D 2 O. The 13 C-NMR spectrum of 5b exhibited the C-3 and C-4 of β-lactam ring at 81.7, 60.2 respectively, C=O (β-lactam) at 165.0 and (C=O, BOC) at 155.5. Subsequent treatment of β-lactams 5a-o with trifluoroacetic acid (TFA) in dry CH 2 Cl 2 at room temperature afforded the deprotected N- Removal of the BOC residue resulted in removing of t-butoxy signal in 1 H-NMR and the appearance of the NH 2 peaks in IR spectra. The removal of the BOC was also confirmed by mass spectra and elemental analyses. Thus, for example the 1 H-NMR spectrum of 6a showed β-lactam H-3 and H-4 protons as two doublets at 5.60 ppm (J = 4.4 Hz) and 5.18 ppm (J = 4.4 Hz), respectively confirming the cis stereochemistry for these free amino β-Scheme 1. Synthesis of cis-β-lactams 5a-o and 6a-o. Anticancer activity Compounds 5c, 5h and 5q-t, were examined for their anticancer properties against K562 Leukemia cell line at two different concentrations (50 and 100 µg/mL) and 5s showed the best activity.

Antimalarial activity
Good to excellent antimalarial activities have been obtained against chloroquine resistant p. falciparum K14 strain as outlined in Table 2 for cis-2-azetidinones 5a-o and β-lactams 6a-o with IC 50 varying from 15 to 50 µM in the better cases. Thus, even if the mechanism of action of these compounds remains unknown some structureactivity relationships can be underlined. Firstly, it is noteworthy that the less active derivatives 5k-5o and 6k-6o differ from the other derivatives by the replacement on the lactam ring of a phenoxy group by a methoxy one suggesting a dramatic contribution of this moiety on the encountered antimalarial activity. On the other hand, a slight change such as removal of the BOC protecting group led to an increase of the biological activity of compounds 6a and 6d (IC 50 = 19 and 22 µM respectively) whereas their protected parent derivatives are totally inactive suggesting here again a quite strong influence of the structure of the considered lactam derivative on the mechanism of action.

Conclusion
In this study, thirty novel β-lactams bearing the N-ethyl tert-butyl carbamate and N-(2aminoethyl) β-lactams were synthesized by [2+2] ketene-imine cycloaddition reaction (Staudinger). The cycloaddition reaction was found to be totally diastereoselective leading exclusively to formation of cis-β-lactam derivatives. These newly synthesized β-lactams were evaluated for their antimalarial activity against p. falciparum K14 resistant strain and showed good to excellent EC 50 values. Of the